Recently, thinner color image display apparatus have been actively developed. As an example, Publication of Unexamined Japanese Patent Application (Tokkai-Hei) No. 3-67444 discloses a flat-type image display apparatus employing a beam scanning method in which the distance from a cathode to an anode is shortened significantly compared with a conventional cathode-ray tube (CRT) system. In the flat-type image display apparatus, a screen is divided into a plurality of subsections vertically. An electron beam is deflected vertically to display a plurality of lines on each subsection. Further, the screen is also divided into a plurality of subsections horizontally. In each subsection, phosphors of R, G, and B emit light sequentially. An amount of the electron beam irradiated onto the phosphors of R, G, and B is controlled by the received color picture signals. Thus, a television picture is displayed as a whole.
In the above-mentioned flat-type image display apparatus, an electrode unit and linear hot cathodes (hereafter referred to as "linear cathodes") as electron beam sources are housed in a flat-box type vacuum case. In the electrode unit, the distance from a cathode to an anode is shortened significantly. Electrodes forming the electrode unit are provided with small holes or slits for deflecting, focusing, and controlling electron beams emitted from the linear cathodes. The electron beams go through the electrodes while being controlled by the holes or slits in each electrode and accelerated to the anode to cause light emission of phosphors applied on the anode, thus displaying images.
FIG. 7 is an exploded perspective view showing the internal configuration of the aforementioned conventional flat-type image display apparatus. The flat-type image display apparatus comprises a back electrode 1, linear cathodes (in the figure, only four linear cathodes are shown) 2 extending horizontally, and an electrode unit 11 including signal control electrodes, which provides the main fictions of the apparatus. The signal control electrodes comprise an electron beam extracting electrode 3 and other electrodes 4-8 for, for example, focusing and deflecting electron beams. The sheet-shaped electrodes 3-8 are superposed via insulators and spacers, thus forming the electrode unit 11. In the electron beam extracting electrode 3, electron beam extracting holes 12 are formed. Electron beams 13 emitted from the linear cathodes 2 are extracted through the holes 12 so as to form an apparent one electron beam per hole. An extracted electron beam 13 is controlled, focused, and deflected by the electrodes 4-8 to scan a subsection 14 on the anode screen. The figure shows only one electron beam 13. However, the same number of electron beams as that of many electron beam extracting holes 12 are extracted from the holes 12.
A front case is formed of a flat-box type front glass case 9. The phosphors of R, G, and B are applied on the inner face of the front glass case 9 by being printed on subsections 14-16 forming the screen. Further, a metal-backed layer is formed on the subsections 14-16 to apply high voltage. The electron beams are accelerated to have high energy and strike the metal-backed layer, thus exciting the phosphors so that the phosphors emit light. The electron beam 13 allows the subsection 14 to emit light to display an image. Similarly, other electron beams that are not shown in the figure cause light emission of a subsection 16 and others. Thus, light is emitted from all of the subsections to display images. Consequently, a desired image is displayed on the screen as a whole. The back electrode 1 is formed on a rear case 10. The rear case 10 and the front glass case 9 are combined and sealed, and then a vacuum is drawn on its inside, thus forming a flat-type image display apparatus.
FIG. 8 is a perspective view showing the appearance of a sealed flat-type image display apparatus. The front glass case 9 and the rear case 10 are baked to be sealed with low melting point glass thus forming a case. The front glass case 9 is provided with an exhaust pipe 17 for drawing the vacuum inside the case, a high-voltage terminal 18 of the anode, and outgoing terminals 19 for controlling various electrodes forming the electrode unit. By connecting a driving circuit, a signal processing circuit, or the like to the terminals externally, the flat-type image display apparatus functions as a television receiver or a display unit.
Internal components constructing the aforementioned flat-type image display apparatus are exposed to high temperature repeatedly in the fabrication and assembling process of the apparatus or in operation of the apparatus for displaying images. For instance, with respect to the fabrication and assembling process, the apparatus is exposed to high temperature in bonding a plurality of fixing platforms for attaching various electrodes onto the glass rear case using low melting point glass and in a baking process of combining and bonding the front case and the rear case. That is to say, for example, the low melting point glass applied on a peripheral adhering portion of a glass case is melted at about 500.degree. C. to seal the glass case, and a process of drawing high vacuum inside the glass case after sealing the glass case is carried out in a heating furnace at about 300.degree.-350.degree. C. On the other hand, in the operation of the apparatus, a number of linear hot cathodes stretched in a plane are heated to a high temperature of 600.degree.-700.degree. C. for generating electron beams. Due to the heat radiation by the linear cathodes, the various internal electrodes also are exposed to the above-mentioned high temperature.
In order that a proper beam spot scans precisely the printed phosphor surface of the screen to avoid deviation of beam position on the screen so as to display vivid images with high precision, the apparatus must be assembled with a precision on a micron level and the precision must be maintained in the operation for displaying images. However, generally objects exposed to high temperature repeatedly are subjected to thermal deformation such as expansion and contraction repeatedly due to the temperature change. Therefore, the high temperature atmosphere and the maintenance of the high precision are physically incompatible with each other.
The problem in a conventional technique will be explained more specifically with reference to FIG. 9. FIG. 9 is a partially enlarged schematic perspective view showing conventional configurations for stretching linear cathodes and for fixing an electrode unit comprising various electrodes. A plurality of linear cathodes 2 are welded and fixed to (vertical type) springs 27 for stretching the linear cathodes 2. Although only one end portion is illustrated in FIG. 9, the linear cathodes 2 are supported at both ends by the springs 27. Thus, suitable tension is applied to the linear cathodes 2 to stretch them without looseness. In addition, the linear cathodes 2 are in contact with and are supported by guide surfaces of horizontal positioning protrusions 25 for horizontally positioning linear cathodes and vertical positioning protrusions 26 for vertically positioning linear cathodes. The horizontal positioning protrusions 25 and vertical positioning protrusions 26 are formed on the supporting platform 24 for stretching the linear cathodes 2. The guide surfaces of the horizontal and vertical positioning protrusions 25 and 26 are manufactured with high precision. Thus, the linear cathodes 2 are positioned with high precision.
Each linear cathode 2 stretched with high precision is positioned so as to pass the center of the electron beam extracting holes 12 manufactured with high precision in the electron beam extracting electrode 3. Then, the electrode unit 11 is fixed with screws 20. In an ordinary condition without temperature change, no problem is caused if all the components have the same thermal expansion coefficient and the temperature does not vary in the respective components. However, in the conventional example, the supporting platform 24 is made of ceramic (alumina) and a plurality of electrode plates forming the electrode unit are formed of iron. Therefore, due to the difference in their thermal expansion coefficients, even if various components are not thermally deformed in a heat process of the fabrication, in the operation of the image display apparatus, the linear cathodes 2 are deviated from the center of the electron beam extracting holes 12 by about 100 .mu.m in a 14-inch apparatus (an object value of the deviation is 20 .mu.m or less). As a result, it is difficult to focus and deflect electron beam uniformly. Consequently, the basic function as an image display apparatus cannot be obtained.
Further, since the supporting platform 24 is required to be manufactured with high precision, grinding and polishing process requiring many steps have to be carried out. Therefore, one more problem is that the cost reduction is difficult.